This invention relates to end seals for laser tubes and particularly to transmissive end windows having enhanced stability and improved thermal properties.
Heretofore, it has been common to manufacture certain gas laser tubes with Brewster window end seals through which radiation developed within the tube is taken for performing useful functions. Such windows have been located at the Brewster angle in order to enhance the efficiency of the tube and prevent unwanted reflections. Materials for such windows have included fused silica. Such fused silica windows can be hard sealed to a laser plasma tube made of the same material by spreading a thin layer of glass frit to the end of the tube to which the fused silica window is impressed so that the glass sealant interposed between the silica tube and the silica window is compressed into a very thin layer upon application of heat. It is also possible to attach such windows by epoxy.
It is found that UV emissions within the tube tend to darken fused silica in the same manner that it is darkened by solarization. Such darkening may be very small and even unobvious to the naked eye, but nevertheless produces an absorption which in turn results in thermal heating of the window. Furthermore, thin films of contamination are often developed on the inner surface of the window which can also cause absorption and heating. In general, the thermal conductivity of fused silica is not adequate to permit dissipation of the heat caused by absorption from the foregoing causes before thermal gradients of such magnitude are created with fused silica windows as to cause undesirable optical gradients and changes in its optical properties. Specifically, the thermal gradients change the index of refraction slightly producing distortion in the wave front of the resulting beam and degradation of mode purity and also causes additional loss of power through diffraction of the beam as it passes out of the laser cavity. There is, therefore, a need for a new and improved end window for use in terminating the light output end of laser plasma tubes.